This proposal is a request for NIH support for a Workshop on use of porphyrins for tumor localization and photosensitization. During the past several years, the porphyrin product HPD has been found capable of concentrating in tumor loci. Neoplastic lesions can be identified by the resulting fluorescence, and selectively destroyed, via porphyrin-catalyzed energy transfer, upon irradiation with red light. Periodic conferences have proved useful for exchange of information among workers in the field, and for providing updates of pre-clinical and clinical results for the benefit of individuals who are beginning research on photosensitization phenomena. The past two such conferences were held in September 1981 (with NIH support) and April 1983. These were successful meetings, providing a useful format for the goals outlined above. During the next such meeting, it is proposed that workers from clinical and preclinical research groups be invited, along with experts in various phases of porphyrin chemistry and physics. A continued exchange of information seems essential in order that progress in this new anti-tumor modality be maintained.